Conventionally, there has been an image forming apparatus which develops an image by causing toner to adhere to an electrostatic latent image formed on a photosensitive drum (image carrier) and transfers a resultant toner image onto transfer paper (transfer material) which is caught on a transfer drum (transfer material carrier).
Such an image forming apparatus is arranged, for example, as follows, as illustrated in FIG. 19: a corona charger 102 which attracts a transfer material P, and a corona charger 104 which transfers a toner image formed on a surface of a photosensitive drum 103 onto the transfer material P, are discretely provided inside a transfer drum which is composed of a cylinder 101 covered with a dielectric layer 101a, so that the attraction of the transfer material P and the transfer are carried out by the chargers 102 and 104, respectively.
Another image forming apparatus of this type, as illustrated in FIG. 20, has (1) a transfer drum which is a two-layer cylinder 201 composed of an outer semi-conductive layer 201a and an inner foundation layer 201b, and (2) a grip system 202 for holding, along a circumferential surface of the cylinder 201, a transfer material P which has been transported thereto. This image forming apparatus is arranged so that an edge of the transfer material P thus arriving there is caught by the grip system 202 and is held on the cylinder 201 around its circumferential surface, and thereafter, the surface of the cylinder 201 is charged by applying a voltage to the outer semi-conductive layer 201a of the cylinder 201 or causing a charger inside the cylinder 201 to discharge electricity, so that the toner image on the photosensitive drum 103 is transferred onto the transfer material P.
However, the image forming apparatus shown in FIG. 19 has a following problem: since the cylinder 101 has a single-layer structure, equipped with only the dielectric layer 101a, the corona chargers 102 and 103 inside the cylinder 101 are indispensable, and as a result this sets a limit to the size of the image forming apparatus when reducing the size is attempted.
In the case of the image forming apparatus shown in FIG. 20, the number of chargers can be decreased since the cylinder 201 has the two-layer structure so that the charging of the cylinder 201 for transferring the toner image onto the transfer material P is facilitated. However, the grip system 202 provided in the image forming apparatus makes the arrangement of the apparatus as a whole complicated, and causes the number of parts used in the apparatus to increase. As a result, a cost for manufacturing the apparatus increases.
Then, as an image forming apparatus which does not have the above problems, the Japanese Publication for Laid-Open Patent Application No. 2-74975/1990 (Tokukaihei 2-74975) discloses an image forming apparatus having (1) a transfer drum composed of a grounded metal roll on which a conductive rubber and a dielectric film are laminated, and (2) a corona charger driven by a unipolar power source, which is provided in the vicinity of a position on the transfer drum where a transfer sheet is separated from the transfer drum.
In the image forming apparatus described above, the transfer sheet is caused to adhere to the transfer drum by inducing charges in the dielectric film with the use of the corona charger. Then, the adhesion of the transfer sheet further causes induction of electric charges, thereby causing transfer.
Therefore, by thus arranging the image forming apparatus, only one charger is required since the charging of the transfer drum surface for adhesion and transfer with respect to the transfer sheet is carried out with the use of the single charger, and the reduction of the transfer drum size can be achieved. Besides, such a system as the aforementioned grip system 202 for holding the transfer sheet is unnecessary. Thus, adhesion of the transfer sheet can be achieved in a simple arrangement.
In the image forming apparatus disclosed by the aforementioned publication, however, the following problem arises. The surface of the transfer drum is charged by atmospheric discharge of the corona charger, and in the case where a color image is formed, that is, in the case where the transfer process is repeatedly carried out several times, electric charges should be supplied by the corona charger every time the transfer process is carried out. Therefore, a charging unit including a unipolar power source or the like for controlling the operation for driving the corona charger is required, and this causes the number of parts constituting the image forming apparatus to increase, thereby resulting in a problem of an increase in the manufacturing cost of the apparatus.
Moreover, if the surface of the transfer drum is scarred, an electric field generated by the atmospheric discharge becomes smaller, and an electric field balance is therefore easily distorted at the scars. Therefore, transfer defects such as voids occur at the scars, and as a result the image quality degrades.
Furthermore, since the surface of the transfer drum is charged by the atmospheric discharge, a high voltage is required for the charging, and as a result energy required for driving the image forming apparatus increases. Besides, since the atmospheric discharge is easily affected by ambient conditions such as humidity of the atmosphere, surface potentials of the transfer drum tend to vary, thereby causing the transfer drum to fail to attract the transfer sheet, and causing distortion of printed pictures and letters.
To solve such problems, the Japanese Publication for Laid-Open Patent Application No. 5-173435/1993 (Tokukaihei 5-173435) proposes a transfer device which has a transfer drum composed of a resilient layer made of an aerated material and a dielectric layer covering the resilient layer, and forms a color image on a transfer material by sequentially transferring uni-color toner images which are sequentially formed on a photosensitive drum, onto the transfer material such as a transfer sheet which adheres to the transfer drum, so that the toner images fall on one another.
In the foregoing transfer device, an attracting roller as charging means is used for causing the transfer material to electrostatically adheres to the transfer drum. Besides, cavities are provided between the resilient layer and the dielectric layer in the transfer drum so that electric charges are accumulated on a reverse surface of the dielectric layer so that ambient conditions may not affect the maintenance of electric charges. By doing so, attracting capacity, that is, an attracting property with respect to the transfer material is improved.
However, as to the arrangement disclosed by Tokukaihei 5-173435, the publication does not particularly specifies a hardness of the aerated layer and a contact pressure (nip pressure) exerted between the attracting roller and the transfer drum, and besides, has no description on a nip width and a nip period. Therefore, it can be considered that the nip period is not variable.
It is generally known that a quantity of electric charges, which are held during a certain period (nip period) by a transfer material while passing through between the transfer drum and the attracting roller, varies with a type of the transfer material. For this reason, a transfer electric field for electrostatic transfer from the photosensitive drum to the transfer material considerably varies with the type of the transfer material. More specifically, in the case where the nip period is set constant, the quantity of electric charges supplied during the period differ depending on types of transfer materials, and the electrostatic transfer capacity of the transfer drum deteriorates in cases of some types of transfer materials. As a result, in such cases, the toner images formed on the photosensitive drum cannot be electrostatically transferred onto the transfer materials in good conditions.
As already known, during the reversal developing method, toner adheres to exposed portions of the photosensitive drum. Background portions of the photosensitive drum have high potentials even after the development, and a transfer currency is great on the transfer of toner to a transfer material. Therefore, the transfer drum has a great attracting force with respect to the transfer sheet. As a result, in the separation process after the transfer, the toner image which has been transferred onto the transfer sheet becomes unstable, or the toner comes off and discharges electricity, thereby scattering on the transfer sheet.
To solve the above-described problem, removing residual charges in the background portions of the photosensitive drum is attempted by exposing the whole surface of the photosensitive drum before the transfer and after the development, in an arrangement disclosed by the Japanese Publication for Laid-Open Patent Application No. 55-17111/1980 (Tokukaisho 55-17111). By doing so, the potentials of the background portions to which toner adheres are lowered, and as a result it is possible to improve the separating operation. However, this also raises a potential of toner on the photosensitive drum, thereby causing scatter of the toner in a horizontal direction (thrust direction).
Note that the scatter of toner signifies distortion of a toner image on the photosensitive drum which is to be transferred, or distortion of toner images to be thereafter subsequently transferred onto the transfer sheet, which occurs on the transferring occasion. To be more specific, the scatter of toner indicates the following phenomenon: for example, in the case where a letter "I" is transferred, toner scatters around the letter "I" on the transfer sheet, thereby resulting in that the transferred letter becomes thicker than an intended thickness.
The aforementioned phenomenon of the scatter of toner is conspicuous in the case where several color toners are laminated so as to form a color image. For example, in the case where a blue letter is formed, a toner image of cyan which has been first transferred is overlapped by a toner image of magenta. In this case, the toner of magenta sometimes scatters around the toner image of cyan.
In the case where an image is developed at a charge quantity of about 10 to 20 .mu.C/g in about three layers of toners with the use of toners whose particles have a diameter of about 10 .mu.m, a potential of one hundred and several tens to three hundred volts is detected on the photosensitive drum. An effective transfer electric field varies by this potential.
The Japanese Publications for Laid-Open Patent Applications No. 1-191168/1989 (Tokukaihei 1-191168), No.1-191169/1989 (Tokukaihei 1-191169), No.1-191172/1989 (Tokukaihei 1-191172), and No.1-191174 to 1-191177/1989 (Tokukaihei 1-191174 to 1-191177) disclose a method of removing charges in the backgrounds of toners by projecting luminous components with wavelengths which pass through the toners. This method is applicable to both the reversal development type and the regular development type. The above publications also examine a method wherein electricity with the same polarity as that of the background potential or a polarity reverse to the background polarity is discharged, and a method for pre-charging toner, as well as a method for controlling a potential of the photosensitive drum.
However, the techniques disclosed by the aforementioned publications are not intended to be applied with respect to a so-called solid transfer body for causing a transfer sheet to adhere to the transfer drum. Therefore, the image forming apparatuses disclosed by the above publications are arranged so that, in the case where a color image is formed, uni-color images are developed on the photosensitive drum so that they overlap each other, and the color toner image thus formed on the photosensitive drum is transferred onto a transfer sheet.
On the other hand, in the case where a solid transfer body is used, or, particularly in the case of transfer by laminating toner images (hereinafter referred to as laminating transfer), the photosensitive drum and a transfer material are brought into contact every time a transfer operation is carried out. Therefore, a surface potential of the transfer material is raised by the background potential of the photosensitive drum, and the effective transfer electric field accordingly becomes smaller, as the transfer operation is repeated twice or three times in the laminating transfer process. This problem stems from that a material of the solid transfer body is a high-resistant material and transmits a small electric currency, thereby having a property of maintaining a potential. An intermediate transfer body made of the same material has the same problem.
Furthermore, the phenomenon that the effective transfer electric field becomes smaller is conspicuous in the case where a transfer material with a high surface resistivity, such as OHP or coated paper, is used. In the case of OHP, a surface potential of OHP on transfer of the second color differs from that on transfer of the first color in a manner such that the transfer electric field lowers by about 300 V to 400 V. For example, in the case where a voltage of 2200 V as a transfer bias voltage for OHP is applied to the solid transfer body, the transfer potential becomes 1500 V on the transfer of the first color since 700 V is lost in attracting OHP. Thereafter, it becomes about 1100 V on the transfer of the second color, and then, becomes about 700 V on the transfer of the third color. Since a lower limit of the transfer potential is found to be 1000 V from experiments, toner of the third color and those which are to be subsequently transferred rather go back to the photosensitive drum side.